The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The sound emissions generated by the turbojet engines of an aircraft are particularly intense at the time of take-off, while the aircraft is generally in the vicinity of inhabited areas.
Numerous research relating to the way to reduce the sound emissions generated by the turbojet engines of aircraft has been conducted in recent years.
This research led, in particular, to the setting up of acoustic absorption panels in the nacelle surrounding the turbojet engine, in particular in the areas where the sound emissions are the most important.
These panels generally operate according to the principle of the Helmholtz resonators, and for this purpose comprise a set of cavities sandwiched between, on the one hand, a solid skin and, and on the other hand, a perforated skin.
The perforated skin is facing the noise emission area, so that the acoustic waves can penetrate through the openings of the perforated skin inside the cavities. The acoustic energy is dissipated by the visco-thermal effect in the cavities. The height of the cavities allows opting for the target frequency band.
Besides their acoustic attenuation function, these panels provide two other functions:
an aerodynamic function: the perforated skin being in contact with the air and gas flows circulating through the turbojet engine and the nacelle, the perforated skin channels the flow and disturb the least possible these flows;
a force absorption function: by means of the sandwich, the acoustic attenuation panel is capable of taking some of the forces undergone by the nacelle.
One of the disadvantages of such panels is that they have a large thickness, which makes their integration difficult in nacelles with increasingly thin lines.
This difficulty is increased for the nacelles adapted to turbojet engines with a high bypass ratio, in which the acoustic frequencies to be absorbed are lower, thus involving even thicker absorption panels. Indeed, the range of attenuation frequencies is directly related to the height of the cavities: the higher this height is, the more the attenuation peak shifts towards the low frequencies.
The improvement of the acoustic attenuation panels thus aims to increase the length of the cavities in order to effectively attenuate the low frequencies, without increasing the total thickness of the panel by reducing the perforated surface of the acoustic skin, while preserving the structural properties of the panel.
In order to increase the length of the acoustic cavities, it is known from document WO 92/12854 to propose an acoustic panel including an alveolar core whose cavities are inclined relative to the direction normal to the skins of the panel. However, this solution has the disadvantage of greatly reducing the mechanical strength of the panel, in particular its strength to the compression forces.
Moreover, it is known from document WO 2011/034469 a solution consisting in gathering several alveoli communicating with each other. A single alveolus per group communicates with the outside of the panel, so that the path traveled by the sound waves is a function of the number of communicating alveoli. However, this solution has the disadvantage of reducing the number of perforations, which reduces the acoustically processed surface.